1. Technical Field
The present disclosure relates to a wiring board for a fingerprint sensor.
2. Background
FIG. 6 illustrates a conventional wiring board 20 for a fingerprint sensor. The conventional wiring board 20 for the fingerprint sensor includes an insulating board 11, a wiring conductor 12, and a solder resist layer 13. The conventional wiring board for a fingerprint sensor is disclosed in Japanese Unexamined Patent Publication No. 2001-46359, for example.
The insulating board 11 has a structure in which a buildup insulating layer 11b is laminated on each of upper and lower surfaces of a core insulating layer 11a. The core insulating layer 11a is made of thermosetting resin containing glass cloth. A thickness of the core insulating layer 11a is 30 μm to 400 μm. The core insulating layer 11a has a plurality of through-holes 14. The buildup insulating layer 11b is made of thermosetting resin not containing glass cloth. A thickness of the buildup insulating layer 11b is 10 μm to 20 μm. The buildup insulating layer 11b has a plurality of via-holes 15.
The wiring conductor 12 is adhered to the upper and lower surfaces of the core insulating layer 11a, an inner side of the through-hole 14, a surface of the buildup insulating layer 11b, and an inner side of the via-hole 15. The wiring conductor 12 is made of plated copper. A thickness of the wiring conductor 12 is 10 μm to 20 μm.
The wiring conductor 12 formed on the uppermost layer partially serves as a fingerprint reading outer strip-shaped electrode for reading a fingerprint 16. As illustrated in FIG. 7, each of the outer strip-shaped electrodes 16 has a thin strip-shaped pattern with a land at its end, and they are arranged parallel to each other along a first direction. A width of a strip-shaped pattern portion of the outer strip-shaped electrode 16 is 5 μm to 20 μm. A distance between the adjacent strip-shaped pattern portions of the outer strip-shaped electrodes 16 is 50 μm to 65 μm.
The wiring conductor 12 positioned next to that on the uppermost layer across the upper buildup insulating layer 11b, that is, the wiring conductor 12 formed on the upper surface of the core insulating layer 11a partially serves as a fingerprint reading inner strip-shaped electrode for reading a fingerprint 17. As illustrated in FIG. 8, each of the inner strip-shaped electrodes 17 has a thin strip-shaped pattern with a land at its end, and they are arranged parallel to each other along a second direction perpendicular to the first direction. A width of the strip-shaped pattern portion of the inner strip-shaped electrode 17 is 30 μm to 65 μm. A distance between the adjacent strip-shaped pattern portions of the inner strip-shaped electrodes 17 is 15 μm to 40 μm.
Furthermore, a plurality of projection electrodes 18 are formed on an upper surface of the inner strip-shaped electrode 17 and each of the projection electrodes 18 projects toward a space between the outer strip-shaped electrodes 16. As illustrated in FIG. 9, a top portion of the projection electrode 18 is positioned between the outer strip-shaped electrodes 16. As for the top portion, a dimension along the first direction is 30 μm to 65 μm, a dimension along the second direction is 30 μm to 45 μm, and a distance to the outer strip-shaped electrode 16 is 10 μm to 20 μm. The top portion is connected to the inner strip-shaped electrode 17 through the via-hole 15a having a diameter of 20 μm to 40 μm. The outer strip-shaped electrodes 16 and the inner strip-shaped electrodes 17 vertically overlap and intersect with each other in perpendicular directions, as illustrated in FIG. 9.
The wiring conductor 12 formed on the lowermost layer partially serves as an external connection pad 19. The predetermined external connection pad 19, outer strip-shaped electrode 16, and inner strip-shaped electrode 17 are connected to each other through the wiring conductors 12.
The solder resist layer 13 is adhered to cover the upper and lower buildup insulating layers 11b and the wiring conductors 12 formed on their surfaces. The solder resist layer 13 is made of thermosetting resin. The solder resist layer 13 contains dispersed silica powder as a filler. A thickness of the solder resist layer 13 from the surface of the wiring conductor 12 is 5 μm to 20 μm. The upper solder resist layer 13 completely covers the wiring conductor 12. The lower solder resist layer 13 has an opening to expose the external connection pad 19.
When a finger is put on an upper surface of the wiring board 20 for the fingerprint sensor and a voltage is applied to the outer strip-shaped electrode 16, electrostatic capacitance is formed between a finger surface and the outer strip-shaped electrode 16 which are opposed across the upper solder resist layer 13. This electrostatic capacitance is high at a projected portion of the fingerprint, and low at a recessed portion of the fingerprint. Thus, a difference in the electrostatic capacitance is detected by sequentially scanning the plurality of outer strip-shaped electrodes 16 and the plurality of inner strip-shaped electrodes 17 while applying a voltage to them, and the fingerprint can be read by processing the detected difference with an external processor. According to the wiring board 20 for the fingerprint sensor, the projection electrode 18 projects and reaches the space between the outer strip-shaped electrodes 16. As a result, the electrostatic bond can be increased between the outer strip-shaped electrode 16 and the inner strip-shaped electrode 17, which improves fingerprint reading sensitivity.
A description will be given to a method for forming the wiring conductor 12 including the outer strip-shaped electrode 16 and the projection electrode 18 in the wiring board 20 for the fingerprint sensor. As illustrated in FIG. 10A, the wiring conductor 12 including the inner strip-shaped electrode 17 is formed on the upper and lower surfaces of the core insulating layer 11a and the inner side of the through-hole 14. The wiring conductor 12 is formed by a known method such as a subtractive method or a semi-additive method.
Next, as illustrated in FIG. 10B, the buildup insulating layer 11b is laminated on each of the upper and lower surface of the core insulating layer 11a having the wiring conductor 12. Then, as illustrated in FIG. 10C, the via-holes 15 including the via-hole 15a are formed in the buildup insulating layers 11b. The via-hole 15 is formed by laser processing. The via-hole 15a is formed above the inner strip-shaped electrode 17 and has a diameter of 20 μm to 40 μm. The via-hole 15 is formed above the wiring conductor 12 not serving as the inner strip-shaped electrode 17 and has a diameter of 50 μm to 70 μm.
Next, a base metal layer (not illustrated) having a thickness of 0.1 μm to 1 μm is formed on the surface of the buildup insulating layer 11b and the inner surface of the via-hole 15 by a method such as an electroless plating method. Then, as illustrated in FIG. 10D, a plating resist layer R12 is formed on each surface of the upper and lower buildup insulating layers 11b. The plating resist layer R12 has an opening pattern corresponding to a wiring pattern of the wiring conductor 12 to be adhered to the surface of the buildup insulating layer 11b. The plating resist layer R12 is formed such that a photosensitive thermosetting resin film is attached to the surface of the buildup insulating layer 11b, exposed and developed to have the predetermined opening pattern, and then thermally cured.
Next, as illustrated in FIG. 10E, to form the wiring conductor 12 including the outer strip-shaped electrode 16 and the projection electrode 18, an electrolytic copper plated layer is adhered to the base metal layer (not illustrated) exposed from the opening pattern of the plating resist layer R12. Next, as illustrated in FIG. 10F, the plating resist layer R12 is removed, and etching is performed to remove the base metal layer (not illustrated) exposed from the electrolytic copper plated layer serving as the wiring conductor 12, whereby the outer strip-shaped electrode 16 and the projection electrode 18 are formed as the wiring conductor 12.
However, according to the conventional wiring board 20 for the fingerprint sensor, as described above, the distance between the adjacent strip-shaped pattern portions of the outer strip-shaped electrodes 16 is 50 μm to 65 μm. In addition, the distance between the top portion of the projection electrode 18 and the strip-shaped pattern portion is 10 μm to 20 μm. Therefore, a thickness of a wall of the opening pattern provided in the plating resist layer R12 which is used to form the outer strip-shaped electrode 16 and the projection electrode 18 is extremely as small as 10 μm to 20 μm.
Thus, when the thickness of the wall of the opening pattern in the plating resist layer R12 is as small as 10 μm to 20 μm, the plating resist layer R12 is likely to be removed or lifted from the base metal layer at a region having the small thickness wall. When the plating resist layer R12 is removed or lifted, the electrolytic copper plated layer emerges at that region, and the electrolytic copper plated layer remains between the outer strip-shaped electrode 16 and the projection electrode 18. As a result, electric insulating reliability is reduced between the outer strip-shaped electrode 16 and the inner-strip-shaped electrode 17.